"Nori" is dried seaweed that is then chopped and pressed into sheets that are used for sushi. "Nori" is also laced with soy sauce that acts like a glue that holds the sushi roll together. Seems the fish like the salty treat.

I've opted to feed my fish straight dried seaweed, like the kind that goes into miso soup. I think it's more 'natural' than "nori". Personal preference.